This paper compares the effectiveness of two biofeedback mechanisms to promote acquisition and transfer of deep-breathing skills using a casual videogame. The first biofeedback mechanism, game adaptation, delivers respiratory information by altering an internal parameter of the game; the second, visual biofeedback, displays respiratory information explicitly without altering the game. We conduct a user study that examines visual biofeedback and game adaptation as independent variables with electrodermal activity, heart rate variability, and respiration as dependent variables. In particular, we evaluate these forms of biofeedback by their ability to facilitate acquisition of relaxation skills and promote skill transfer to subsequent stressful tasks. Our results indicate that game adaptation promotes skill acquisition and transfer more effectively than visual biofeedback, but that a combination of the two outperforms either in isolation. Combining visual and game biofeedback also results in faster learning of deep-breathing skills than either channel alone. Our study suggests that the two forms of biofeedback play different roles, with game adaptation being more effective in encouraging deep breathing, and visual channels helping players maintain the target breathing rate.